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/*
Copyright (c) 2021, MariaDB Corporation.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; version 2 of the License.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1335 USA */
#include "sql_statistics.h"
/*
An equi-height histogram which stores real values for bucket bounds.
Handles @@histogram_type=JSON_HB
Histogram format in JSON:
{
// The next three are saved but not currently analyzed:
"target_histogram_size": nnn,
"collected_at": "(date and time)",
"collected_by": "(server version)",
"histogram_hb": [
{ "start": "value", "size":nnn.nn, "ndv": nnn },
...
// Optionally, start and/or end can be replaced with _hex variant
{ "start_hex: "value", "size":nnn.nn, "ndv":nnn},
...
{ "start": "value", "size":nnn.nn, "ndv": nnn, "end": "value"},
]
}
Histogram is a JSON object. It has some global properties and "histogram_hb"
member whose value is a JSON array of histogram buckets.
Each bucket is an object with these members:
"start" - the first value in the bucket.
"size" - fraction of table rows that is contained in the bucket.
"ndv" - Number of Distinct Values in the bucket.
"end" - Optionally, the last value in the bucket.
A bucket is a single-point bucket if it has ndv=1.
Most buckets have no "end" member: the bucket is assumed to contain all
values up to the "start" of the next bucket.
The exception is single-point buckets where last value is the same as the
first value.
start/end can be replaced with start_hex/end_hex. In _hex variant, the
constant is encoded in hex. This encoding is used to handle so called
"unassigned characters": some non-UTF8 charsets have byte combinations that
are not mapped to any UTF8 character.
*/
class Histogram_json_hb final : public Histogram_base
{
size_t size; /* Number of elements in the histogram */
/* Collection-time only: collected histogram in the JSON form. */
std::string json_text;
struct Bucket
{
// The left endpoint in KeyTupleFormat. The endpoint is inclusive, this
// value is in this bucket.
std::string start_value;
// Cumulative fraction: The fraction of table rows that fall into this
// and preceding buckets.
double cum_fract;
// Number of distinct values in the bucket.
longlong ndv;
};
std::vector<Bucket> buckets;
std::string last_bucket_end_endp;
public:
static constexpr const char* JSON_NAME="histogram_hb";
bool parse(MEM_ROOT *mem_root, const char *db_name, const char *table_name,
Field *field, const char *hist_data,
size_t hist_data_len) override;
void serialize(Field *field) override;
Histogram_builder *create_builder(Field *col, uint col_len,
ha_rows rows) override;
// returns number of buckets in the histogram
uint get_width() override
{
return (uint)size;
}
Histogram_type get_type() override
{
return JSON_HB;
}
/*
@brief
This used to be the size of the histogram on disk, which was redundant
(one can check the size directly). Return the number of buckets instead.
*/
uint get_size() override
{
return (uint)size;
}
void init_for_collection(MEM_ROOT *mem_root, Histogram_type htype_arg,
ulonglong size) override;
double point_selectivity(Field *field, key_range *endpoint,
double avg_sel) override;
double range_selectivity(Field *field, key_range *min_endp,
key_range *max_endp, double avg_sel) override;
void set_json_text(ulonglong sz, const char *json_text_arg,
size_t json_text_len)
{
size= (size_t) sz;
json_text.assign(json_text_arg, json_text_len);
}
private:
int parse_bucket(json_engine_t *je, Field *field, double *cumulative_size,
bool *assigned_last_end, const char **err);
double get_left_fract(int idx);
std::string& get_end_value(int idx);
int find_bucket(const Field *field, const uchar *lookup_val, int *cmp);
};
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